In 3GPP (Third Generation Partnership Project), a technology that uses a relay device (relay station) to realize an increase in the throughput at the cell edge is being actively considered.
This relay device receives, in a downlink, a signal transmitted from a base station, amplifies the same, and then transmits the amplified signal to a mobile terminal. By performing such relaying, the relay device can increase the signal-to-noise ratio than when directly transmitting a signal from the base station to the mobile terminal. Similarly, in an uplink, the relay device can maintain high signal-to-noise ratio by relaying a signal transmitted from the mobile terminal to the base station. Additionally, such relaying by the relay device is described in Non-Patent Literature 1, for example.
Furthermore, as a relay scheme of the relay device, an Amp-Forward type, a Decode-Forward type, and the like can be cited. The Amp-Forward type is a scheme of amplifying and transmitting a received signal while keeping it as an analogue signal. According to this Amp-Forward type, although the signal-to-noise ratio is not improved, there is an advantage that the communication protocol does not have to be refined. Additionally, the relay device has a feedback path between a transmission antenna and a reception antenna, and is designed such that the feedback path does not oscillate.
The Decode-Forward type is a scheme of converting a received signal to a digital signal by AD conversion, performing decoding such as error correction on the digital signal, encoding again the decoded digital signal, converting the digital signal to an analogue signal by DA conversion, amplifying the analogue signal, and transmitting the same. According to the Decode-Forward type, the signal-to-noise ratio can be improved by a coding gain. Also, by storing a digital signal obtained by reception in a memory and transmitting the digital signal in the next time slot, the relay device can avoid oscillation of a feedback path between a transmission antenna and a reception antenna. Additionally, the relay device is also capable of avoiding the oscillation by changing the frequency instead of the time slot.